Currently, as a broadband optical access technology, the PON is in a point-to-multipoint topology structure.
FIG. 1 is a network structural view of the PON. The PON includes an optical line terminal (OLT) 101, a passive optical distribution network (ODN) 102, and one or more optical network units (ONUs) 103. The OLT 101 is located in a central office, and provides wide area network interfaces upwards, including Gigabit Ethernet (GE), Asynchronous Transfer Mode (ATM), Digital Signal 3 (DS-3) (45 Mbit/s digital rate) interfaces. The ONU 103 is at the user side, and provides a 10/100 BaseT (fast Ethernet), a T1/E1 (T1: 1.5M digital rate, and E1: 2M digital rate), the DS-3, and other application interfaces for the user. The ODN 102 is composed of a branching device/a coupler, and other passive devices, and is connected to one OLT 101 and one or more ONU 103 passively. Upstream data and downstream data are borne by different wavelengths, the downstream data is sent in a broadcast mode, and the upstream data is accessed in a time division multiple access (TDMA) mode based on statistical multiplexing. Therefore, the PON has advantages of shared bandwidth resources, lower machine room investment, high equipment security, a high network construction speed, and low comprehensive networking cost.
With the development of broadband services, the PON technology is continuously evolved from an ATM PON (APON) and a broadband PON (BPON), to an Ethernet PON (EPON) and a Gigabit PON (GPON), and the transmission bandwidth is continuously increased. For a current GPON, the downstream rate is 2.5 Gbps or 1.25 Gbps, and an upstream rate includes various rates, for example, 2.5 Gbps, 1.5 Gbps, and 622 Mbps.
The GPON adopts a new transmission protocol GPON Encapsulation Method (GEM). The GEM originates from a generic framing concept of the Generic Framing Protocol (GFP); meanwhile, in consideration of the multi-ONU, and the multi-port multiplexing of the PON network, a port identity PORT-ID is introduced. Viewed from a service port of the ONU, the port has a point-to-point connection with the OLT, and is identified by the PORT-ID.
As for differences between the GEM and the GFP, the GFP is processed according to 8-bit bytes, a payload length indicator (PLI) of the GFP is 16 bits, and a PLI of the GEM is 12 bits, a head error control (HEC) of the GFP is 16 bits, and a cyclic redundancy code (CRC) of the GEM is 12 bits. Further, the GEM has the PORT-ID, and may be multiplexed or switched according to the PORT-ID, but the GFP does not have a link layer switching capability. In addition, the GEM is used only between the ONU and the OLT in the GPON, and the GFP is only applicable in the transmission network as the adaptation protocol of the transmission channel.
It can be seen from the above description that, the GEM is not only an adaptation protocol, but also a link layer protocol having a switching capability. The protocol may complete adapting the diversified services of the high layer, which includes an ATM service, a time division multiplex (TDM) service, and an Internet protocol (IP)/Ethernet service. The adaptation for the diversified services is efficient and transparent; meanwhile, the protocol supports the multiplexing, a dynamic bandwidth assignment (DBA), and other operation and management (OAM) schemes, which is suitable for encapsulating service data of the Ethernet, and is also suitable for encapsulating the service data of the TDM, and is one of the ideal solutions of the integrated service access.
A frame structure of the GPON has a cycle of 125 microseconds and has a frame synchronization area, a management overhead area, and a payload area. The downstream is successive signals, and the upstream is the TDMA mode. The OLT indicates each ONU upstream time slot in the downstream frame overhead, and each ONU sends a burst data packet according to a time slot position indicated by the downstream frame overhead.
FIG. 2 shows a structure of the GPON downstream frame. As shown in FIG. 2, one downstream frame in the GPON includes a Physical Control Block downstream (PCBd) overhead area and a payload area. An Upstream Bandwidth Map (US BW MAP) domain in the PCBd overhead area is adapted to assign the upstream bandwidth, and includes an upstream time slot indicator overhead indicating a start position and an end position of each ONU upstream time slot. A control object of the bandwidth assignment is a transmission container (T-CONT), the OLT may assign one or more T-CONTs for one ONU, which is a concept introduced in the PON DBA technology to improve an efficiency of the DBA. In FIG. 2, PLend indicates a payload length downstream.
FIG. 3 shows a structure of the upstream frame of the GPON. Each ONU respectively sends the upstream burst data packet to the OLT in the T-CONT assigned by the OLT, in which the upstream burst data packets include an overhead area and a payload area. The overhead area includes a Physical Layer Overhead upstream (PLOu) domain, an Upstream Physical Layer OAM upstream (PLOAMu) domain, a Physical Layer Sequence upstream (PLSu) domain for power adjustment, and a Dynamic Bandwidth Report upstream (DBRu) domain.
FIG. 4 shows a specific structure of the burst data packet sent by a single ONU. The PLOu domain is used for realizing the burst synchronization, and includes a Preamble, a Delimiter, a bit-interleaved parity (BIP) (bit-interleaved parity-8). After occupying the upstream channel, the ONU firstly sends the PLOu unit to the OLT, so that the OLT may be synchronous with the ONU rapidly, and correctly receives valid upstream data of the ONU. The PLOAMu domain is used for bearing the upstream PLOAM information, which includes an ONU ID, a Message ID, a Message, and a CRC.
Next, the GPON is taken for example to describe the working principles of the PON.
FIG. 5 is an application example of the GPON in data backhaul in the conventional art, and a data transmission procedure is described in the following by taking the Ethernet service data transmission as an example.
Downstream Direction
A service network 505 sends an Ethernet data frame to a transmission network 504 through an Ethernet interface.
The transmission network 504 supports the GFP adaptation protocol, encapsulates the Ethernet data frame through the GFP, adapts the GFP frame to a transmission channel, transmits the GFP frame to a corresponding transmission node, recovers the Ethernet data frame from the GFP frame, and transmits the recovered Ethernet data frame to the corresponding OLT 503.
The OLT 503 supports the GEM adaptation protocol, encapsulates the received Ethernet data frame into a GEM frame, maps a plurality of GEM frames to a payload area of a downstream frame, adds a PCBd overhead, in which a US BW MAP indicates an upstream bandwidth assigned for each ONU, and sends the downstream frame to the corresponding ONU.
The ONU 501 extracts the GEM frame from the payload area in the downstream frame, removes the encapsulation of the GEM, recovers the Ethernet data frame, and sends the recovered Ethernet data frame to each access user.
Upstream Direction
The ONU 501 firstly encapsulates the Ethernet data frame sent by the access user into a GEM frame, maps many GEM frames to the payload area of the upstream burst data packet, adds the upstream overhead, forms and encapsulates the upstream burst data packet into the T-CONT, and transmits the upstream burst data packet in an upstream line according to the upstream bandwidth assigned by the OLT 503.
The OLT 503 receives the T-CONT, extracts the upstream overhead, extracts the GEM frame of the payload area, removes the encapsulation of the GEM, and recovers the Ethernet data frame.
The OLT 503 sends the recovered Ethernet data frame to the transmission network 504 through the corresponding Ethernet port.
The transmission network 504 encapsulates the Ethernet data frame through the GFP, adapts the GFP frame into the transmission channel of the transmission network 504, transmits the GFP frame to the corresponding transmission node, decapsulates the GFP frame to recover the Ethernet data frame, and sends the backhaul of the GPON access data to the service network 505.
It can be known from the application examples that, although the PON has a large capacity, the transmission distance is short, the number of switching offices of the service network is limited, and the distances between the OLTs and the switching offices of the service network are different, so that most of the OLTs cannot be directly connected to the service network to implement the data backhaul, and the service data requires another transmission of the transmission network to reach the service network.